[ConstantFolding] Handle leading zero-size elements in load folding

Struct types may have leading zero-size elements like [0 x i32], in
which case the "real" element at offset 0 will not necessarily coincide
with the 0th element of the aggregate. ConstantFoldLoadThroughBitcast()
wants to drill down the element at offset 0, but currently always picks
the 0th aggregate element to do so. This patch changes the code to find
the first non-zero-size element instead, for the struct case.

The motivation behind this change is rust-lang/rust#48627.
Rust is fond of emitting [0 x iN] separators between struct elements to
enforce alignment, which prevents constant folding in this particular case.

The additional tests with [4294967295 x [0 x i32]] check that we don't
end up unnecessarily looping over a large number of zero-size elements
of a zero-size array.

Differential Revision: https://reviews.llvm.org/D55169

llvm-svn=348895

Author: nikic

llvm/lib/Analysis/ConstantFolding.cpp

@@ -347,9 +347,20 @@ Constant *llvm::ConstantFoldLoadThroughBitcast(Constant *C, Type *DestTy,
 
     // We're simulating a load through a pointer that was bitcast to point to
     // a different type, so we can try to walk down through the initial
-    // elements of an aggregate to see if some part of th e aggregate is
+    // elements of an aggregate to see if some part of the aggregate is
     // castable to implement the "load" semantic model.
-    C = C->getAggregateElement(0u);
+    if (SrcTy->isStructTy()) {
+      // Struct types might have leading zero-length elements like [0 x i32],
+      // which are certainly not what we are looking for, so skip them.
+      unsigned Elem = 0;
+      Constant *ElemC;
+      do {
+        ElemC = C->getAggregateElement(Elem++);
+      } while (ElemC && DL.getTypeSizeInBits(ElemC->getType()) == 0);
+      C = ElemC;
+    } else {
+      C = C->getAggregateElement(0u);
+    }
   } while (C);
 
   return nullptr;

llvm/test/Transforms/ConstProp/loads.ll

@@ -269,3 +269,42 @@ define i64 @test16.3() {
 ; BE-LABEL: @test16.3(
 ; BE: ret i64 0
 }
+
+@g7 = constant {[0 x i32], [0 x i8], {}*} { [0 x i32] undef, [0 x i8] undef, {}* null }
+
+define i64* @test_leading_zero_size_elems() {
+  %v = load i64*, i64** bitcast ({[0 x i32], [0 x i8], {}*}* @g7 to i64**)
+  ret i64* %v
+
+; LE-LABEL: @test_leading_zero_size_elems(
+; LE: ret i64* null
+
+; BE-LABEL: @test_leading_zero_size_elems(
+; BE: ret i64* null
+}
+
+@g8 = constant {[4294967295 x [0 x i32]], i64} { [4294967295 x [0 x i32]] undef, i64 123 }
+
+define i64 @test_leading_zero_size_elems_big() {
+  %v = load i64, i64* bitcast ({[4294967295 x [0 x i32]], i64}* @g8 to i64*)
+  ret i64 %v
+
+; LE-LABEL: @test_leading_zero_size_elems_big(
+; LE: ret i64 123
+
+; BE-LABEL: @test_leading_zero_size_elems_big(
+; BE: ret i64 123
+}
+
+@g9 = constant [4294967295 x [0 x i32]] zeroinitializer
+
+define i64 @test_array_of_zero_size_array() {
+  %v = load i64, i64* bitcast ([4294967295 x [0 x i32]]* @g9 to i64*)
+  ret i64 %v
+
+; LE-LABEL: @test_array_of_zero_size_array(
+; LE: ret i64 0
+
+; BE-LABEL: @test_array_of_zero_size_array(
+; BE: ret i64 0
+}